Process for the production of cast iron with spherolites

ABSTRACT

A CAST IRON WITH SPHEROIDAL GRAPHITE IS PRODUCED WITH A RELATIVELY LOW SILICON CONTENT WHICH HAS IMPROVED PLASTICDEFORMATION BEHAVIOR AND TOUGHNESS AT ROOM TEMPERATURES AND BELOW TO-40*C. COBALT AND MAGNESIUM ARE ADDED IN COMMON AND SIMULTANEOUSLY TO ACHIEVE RESIDUAL CONTENTS OF FROM 0.1 TO 1.5% AND FROM 0.001 TO 0.08% BY WEIGHT, RESPECTIVELY.

United States Patent 3,689,255 PROCESS FOR THE PRODUCTION OF CAST IRONWITH SPHEROLITES Erich K. Modl, Winterthur, Switzerland, assignor toSulzer Brothers, Ltd., Winterthur, Switzerland N0 Drawing. Filed Nov.30, 1970, Ser. No. 93,802 Claims priority, application Switzerland, Dec.30, 1969,

Int. Cl. C22c 37/04 U.S. Cl. 75-123 CB Claims ABSTRACT OF THE DISCLOSUREHeretofore, it has been known to treat ferrous based melts which are tobe used in the production of ductile cast iron, i.e. cast ironcontaining spheroidal graphite, with Fe-Si (iron-silicon) master alloyscontaining magnesium (Mg). However, for grades having increasedtoughness, it has been necessary to have as low a silicon content as ispossible since a high silicon content produces brittleness of a ferriticbasic mass.

Further, where a treatment medium has been added to a melt to be used inthe production of cast iron containing spheroidal graphite, in order tomaintain the silicon content of the basic melt substantially unchanged,the treatment medium has been a silicon-free magnesium master alloy,such for example, a Ni-Mg (nickel-magnesium) or Cu-Mg(copper-magnesium). However, the nickel or copper which has thus beenintroduced produces no ferritization but instead favors the pearliteformation. Consequently, these known magnesium containing silicon-freemaster alloys should not be used for the production of ductile castirons which are to possess a ferritic basic structure.

It has been known how to use iron-magnesium (Fe- Mg) master alloys, inthe form of compressed material, as treatment additives for cast ironwith spheroidal graphite. However, the use of Fe-Mg master alloys hasnot been reliable, and the formation of spherolites has often beenimperfect. In this regard, it is noted that an unfavorable shape and asmall number of spherolites lowers the mechanical characteristics of thecastings made from cast iron having such spherolitic graphite. A furtherdrawback of these specifically master alloys having light weights residein their handling because these alloys do not, of themselves, becomesubmerged in the basic ferrous melt, but float upon the surface, andthus impair the magnesium (Mg) yield. When used in an immersion process,the treatment costs become substantially increased.

Accordingly, it is an object of the invention to produce a cast-ironmaterial having a relatively low silicon content and a ferritic basicstructure in a cast state.

It is another object of the invention to form a casting having animproved plasic deformation behavior at room temperature and at lowertemperatures.

Briefly, the invention provides a process for producing a ductile castiron which can be used to form a casting having improvedplastic-deformation behavior and improved notch-impact toughness values.

The process includes the step of adding to an initial melt at leastmetallic cobalt and magnesium in common and simultaneously, inquantities that lead to residual magnesium contents of 0.01 to 0.08% byweight, and to cobalt contents of 0.1 to 1.5% by weight, in the casting.

It is true that it is known how to use cobalt as an alloying element forductile cast iron. However, the cobalt has been added before a magnesiumtreatment has been performed. As a result, in spite of the ferritizingeffect found with a chemical composition comparable to that of a castiron of the invention, particularly one having an equal carbon andsilicon content, and in spite of an increase in the number ofspherolites and an improvement of the spherolite formation, it has beenfound that the subsequent magnesium treatment allowed the familiarundesirable effect of the magnesium carrier to gain influence again. Inaddition to this, where cobalt has been used previously as an alloyingelement the effective cobalt content has been relatively large, i.e. inthe region of approximately 3% to 6% by Weight of cobalt.

Apart from its positive characteristics, which result from the effectsof the ferritic basic structure and the low silicon content, thetreatment process of the invention also has the same advantages asnickel-magnesium or the copper-magnesium treatments, particularly withrespect to the simple manipulation required for introduction into themelt.

By achieving substantailly lower cobalt contents, the advantages foundin investigations with cobalt-alloyed cast iron with spheroidalgraphite, particularly as to mechanical and technological values, areobtained to an increased degree. It has furthermore been found that withsimultaneous cobalt treatment, it is possible to keep the residualmagnesium content needed for the formation of the spherolitesparticularly low. This is of particularly great importance in order toavoid dross and its known unfavorable influence on the quality ofcastings.

In accordance with the invention, the magnesium may be replaced, inpart, by at least one other of the elements known to promote a formationof spherolitic graphite, such for example as calcium, yttrium and/orrare earthmetals, for example, cerium, lanthanum, etc.

In order to promote still lower silicon contents (below 1.3% by weight),it is advantageous to add aluminum in quantities of up to 3% by weightto the melt, at the same time the magnesium and cobalt are added.

Since magnesium is insoluble in its solid state in cobalt, it is notpossible, because of the alloying behavior of these two elements, tosmelt a actual magnesium-cobalt master alloy. The addition of thetreatment medium to the molten ferrous based materials can, however, beaccomplished advantageously by adding compressed and/ or sintered formeditems, briquettes or compressed items, that contain in finely comminutedform at least cobalt and magnesium, and are except for impurities, freeof silicon. Porous cobalt-carrying substances impregnated with magnesiumcan also be added to the molten metal. Also, formed items, produced fromcomminuted or pulverized cobalt and mag nesium by the aid of bindingmediums, such for example, as lime milk or cement, that do not influencethe quality of the melt can be used to introduce the cobalt andmagnesium.

One particular field of application for a ductile cast iron treated witha cobalt-magnesium additive is in the production of a casting that uponsolidification displays an aligned dendritic crystalline growth, suchfor example, as a thin-walled sand casting, a continuous-length castingor a casting made in permanent molds producing a relatively greatshrinkage effect, e.g. a casting made in a chill-mold.

The invention is also applicable to a cast iron with spheroidal graphitehaving a maximum silicon content of 2.1% by weight, a maximumphosphorous content of 0.05% by weight, and a maximum magnanese contentof 0.1% by weight, for making castings having a preponderant ferriticbasic structure and that at room temperature and at lower temperaturesdown to -40 C. display an improved plastic deformation behavior. It ishereby possible to tolerate in the ferritic basic structure, dependingon the required plastic deformability, a pearlite proportion going up toabout 20% as viewed at the surface of the microstructure under a100-times optical enlargement.

The magnesium-cobalt treatment of the invention also allows a casting,apart from special cases, to be used in the cast state without furthertime-consuming and expensive heat-treatments.

The invention is explained in more detail in the following by the aid ofthe following examples.

EXAMPLE 1 In a high-frequency induction crucible furnace of suitablesize, lined with masses of compressed magnesite, about 11 kilograms(kg.) of magnetic iron scrap was melted down and carburized with 0.5kilogram (kg) of graphite.

The basic ferrous melt was of the following chemical composition: carbon(C)=3.7; silicon (Si)=1.80; manganese (Mn) =0.13; phosphorous (P) =0.01;sulphur (S) =0.008.

The basic melt was treated with a cobalt-magnesium additive medium at atemperature of 1480 C., or C. The additive medium had previously beenprepared, by a powder-metallurgical technique, from a mixture of 76%powdered cobalt (99.56% Co content, grain size 40 microns tm.]), 4%cesium (Ce) powder of a similar grain size, and 20% magnesium (Mg)powder (99.8% Mg, grain size 100 MIL), the form of compressed tabletswhich had been cold-pressed with a pressure of 3.5 metric tons persquare centimeters (cm?). The quantity of treatment medium addedamounted to 1% of the weight of the melt. The additive was introducedthrough immersion, by means of a graphite bell, in the molten metal. Thetreatment proceeded smoothly, and without any special light effects. Atabout 1430 C., the treated melt was inoculated with 0.3% Fe-Si (75%),and was then cast into specimens for testing (Specimen Y-block size 2 inper ASTM A536 67) the casting being done in dry slightly preheated sandmolds. Chemical analysis of the specimens showed: Percentages by weight:C=3.84; P=0.01; S=0.007; Si=2.25; Mn=0.15; Mg=0.027; Co=0.50.

The specimens were then investigated in the usual way. Determination ofthe mechanical characteristics yielded the following measured individualvalues: Tensile strength: 47.7 and 48.3 kg./mm. Yield strength: 31.0 and33.2 -kg./mm. Elongation in 2 inches: :18.7 and 18.8%; BHN (Brinellhardness number). 143 and 144 kg./mm.

Investigation of the microstructure showed numerous well-formedspherolites in the preponderatingly ferritic basic structure, thepearlite prooprtion being less than 3% in the cast state.

EXAMPLE 2 A ferrous based melt, smelted as described above and whosesilicon content amounted to approximately 1.7% by weight, was treatedwith a treatment medium made by cold-compression from cobalt, magnesiumand aluminum in the following proportions: 60% cobalt; 20% magnesium;20% aluminum. The treatment temperature of Example 1 was retained.

The specimens, cast at the same casting temperature as Y test-pieces,had the following mechanical characteristics: Tensile strength 49.4kg./mm. Yield strength 34.3 kg./mm. Elongation in 2 in.: 16.8%; BHN: 154kg./ mm. Charpy V-notch impact values: 1.8 ml g./cm. at 20 C.; 1.2mkg./cm. at 40 C.

Investigation of the structure again showed well-formed spherolites in aferritic basic structure having pearlite in a maximum proportion of 5%.

EXAMPLE 3 For the continuous casting of horizontal bars (35 to 43 mm.diameter) from ferrous material with spheroidal graphite having apreponderatingly ferritic basic structure, it is necessary periodicallyto re-treat the basic melt, pretreated with various kinds of Mg masteralloys, in heat retaining containers with Mg-containing additives, forthe purpose of compensating the fading-away effect of the initialmagnesium (Mg) treatment.

When use is made of the usual Si-containing magnesium master alloys,this results in a continuous increase of Si, which impairs theplastic-deformation capacity of the ferritic continuously-cast length.If in accordance with the invention, the Mg after-treatment is done withthe Si-free Mg-Co additive, then no undersirable increase of Si occurs,and the originally optimum Si content can be positively maintained.

The quantity of iron present in each case in the heatretaining containerof the continuous-casting machine is, at intervals of about 1 hour givenan addtion of 400 to 800 kg. of pretreated liquid metal. The treatmentis carried out by the immersion process, with 3.4 kg. of Mg-Co additiveat intervals of 10 to 15 minutes. The treatment temperature is 1340 C.The continuous-casting produced in this way has the following measuredfinal values: Yield strength: 38 to 45 kg./mm. Tensile strength: 43 to65 kg./mm. Elongation in 2 in.: 17% to 21%; BHN: kg./mm.

The microstructure showed across the entire cross-section a very uniformand preponderant ferritic solidification, with perfectly-formed smallspherolites.

By using Mg-Co additives a definite high degree of uniformity of thestructure occured, and. the separating-out of eutectic cementite wassuppressed, while slag and dross inclusions were diminished.

The invention is obviously not limited to the above described examples.It is, of course, also easily possible to increase the already very goodmechanical characteristics of the castings obtained in their cast stateby heat treatment.

What is claimed is:

1. A process for the production of cast iron with spheroidal graphitecomprising the step of adding at least metallic cobalt and metallicmagnesium in common and simultaneously to a melt with the cobalt andmagnesium being added in quantities to produce a residual cobalt contentof from 0.1% to 1.5 by weight and a residual magnesium content of from0.01% to 0.08% by weight.

2. A process as set forth in claim 1 which further comprises the step ofadding at least one element from the group consisting of spheroliteforming elements to the melt in common and simultaneously with thecobalt and magnesium.

3. A process as set forth in claim 2 wherein said elements includecalcium, strontium, barium, scandium, thorium, yttrium and rare earthmetals.

4. A process as set forth in claim 2 which further comprises the step ofsimultaneously adding aluminum in a quantity of up to 3 by weight of themelt with the cobalt and magnesium.

5. A process as set forth in claim 1 which further comprises the step ofsimultaneously adding aluminum in a quantity of up to 3 by weight of themelt with the cobalt and magnesium.

6. A process as set forth in claim 1 wherein the cobalt and magnesiumare in comminuted form within a preformed silicon free item.

7. A cast iron made in accordance with claim 1 for a casting having apreponderant ferritic basic structure containing 2.1% by weight maximumof silicon, 0.05% by weight maximum phosphorous, 0.1% by weightmanganese, 0.1% to 1.5% by weight of cobalt and 0.1% to 0.08% by weightof magnesium and having an improved plastic deformation behavior andimproved notch-impact toughness values at room temperatures and at lowertemperatures down to -40 C.

8. A process for the production of cast iron with spheroidal graphiteconsisting of the step of adding metallic cobalt and metallic magnesiumin common and simultaneously to a ferrous melt, said cobalt being addedin an amount by weight greater than the amount of added magnesium.

9. A process as set forth in claim 8 wherein the metallic cobalt isadded in an amount of from 0.1% to 1.5% by weight of the total weight ofthe resultant melt.

10. A process as set forth in claim 8 wherein the metallic magnesium isadded in an amount of from 0.01% to 0.08% by weight of the total weightof the resultant melt.

References Cited UNITED STATES PATENTS FOREIGN PATENTS 5/1964 GreatBritain.

L. DEWAYNE RUTLEDGE, Primary Examiner J. E. LEGRU, Assistant ExaminerUS. Cl. X.R.

15 7s-130 A, 130 AB UNITED STATES PATENT OFFICE CERTIFICATE OFCORRECTION Patent No. 3: 9, 55 Dated September 5, i 1972 Erich K. ModlInventor(s) It is certified that error appears in the above-identifiedpatent and that said Letters Patent are hereby corrected as shown below:

Claim 1, line after "a" insert --ferrous-- Claim 1, line 7, after"weight. insert --in the resultant Y melt.-- I

Signed and sealed this 13th day of March 1973..

(SEAL) Attest:

EDWARD M.FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissionerof Patents FORM PO-IOSO (10-69) I USCOMM-DC 6O376-P69 u.s. GOVERNMENTPRIN'HNG OFFICE: 1909 o-aes-aaa

